Lysimeter Studies to Investigate the Leaching of Radioactivity From Low-Level Waste

Abstract

Lysimeters are widely used to investigate the release of contaminants from waste or soil samples and for waste degradation studies [1-2]. This paper presents results from the first stage of a comprehensive, long-term lysimeter programme currently being undertaken by BNFL [3] to investigate the release of radionuclides from LLW in lysimeters simulating unsaturated trench disposal conditions. Data generated from these experiments will be used to support mathematical modelling of near field disposal systems.

The three lysimeters used were small-scale, gas-tight columns made from PVC tubing. Two sizes were utilised: the 70-litre lysimeter A was filled with 11.5 kg of waste and the 30-1itre lysimeters B and C contained 5.05 kg and 4.45 kg respectively. Average waste density was 0.17 te/m³. The LLW was taken from R&D labs at the Sellafield site and included materials such as rubber gloves, plastic bottles and bags, paper, card and tissues. Lysimeter B had a higher cellulose content (85%) compared to A and C (50%). After loading, the lysimeters were sealed and fitted with a gas vent to prevent a build-up of internal gas pressure. The lysimeters were operated in the conventional single-pass mode. The irrigation rate of 761 mm per annum was based on the annual rainfall typical for a temperate climate minus potential evaporation. The leachant used was rainwater, which was passed through plastic drums containing rock fragments, soil and gravel to simulate the effect of water passing through an engineered cap. Lysimeter A was operated for 21 months, B and C for 39 months. Prior to the start of irrigation, 30 litres of leachant was passed through lysimeter C to determine the effect of pre-washing the waste.

The input leachant and output leachate were routinely monitored and averaged results are presented in Table I where the range represents two standard deviations from the mean. Typical pH profiles with time exhibited a rapid fall to around 5, followed after 4-5 months by an erratic period of increase before settling in the region 6.3 - 6.4. All the lysimeters exhibited broadly similar redox potential trends: erratic variations but values generally below 250 mV. The major ions except nitrate and sulphate exhibited typical washout behaviour: after an initial peak, concentration levels were similar to the input leachant values. Leachate nitrate levels were below the limit of detection possibly due to a combination of nitrate utilisation by microbes for growth and denitrification bacteria. Leachate sulphate levels also showed a marked decline for all the lysimeters after a few months. This change may be due to Sulphate Reducing Bacteria. From the start of gas sampling in month 2, CO₂levels were high (>20%) and 02 levels were low (<5%), and this pattern was maintained throughout the experiment. Significant methane generation commenced first in lysimeter C (after 5 months), then A (10 months) and finally B (22 months). H₂levels significantly higher than background were also observed up to a peak of 0.18%. The more rapid onset of methanogenesis in lysimeter C has been attributed to the fact that the waste was prewashed prior to the start of routine irrigation. These observed pH, redox and gas composition patterns are consistent with other studies [1] and may be ascribed to the degradation of cellulose in the waste in a series of linked processes: hydrolysis, aerobic respiration, anaerobic fermentation, acetogenesis and methanogenesis.

Radionuclide release plots showed a bimodal pattern which is commonly observed for a range of lysimeter designs and radionuclides including the actmides [2]. There is an imtial peak followed by a long tail when radionuclide concentrations in the leachate are relatively constant over a long time period. The initial peak may be attributed to washout of soluble ionic species from easily accessible sources of contamination. Long term release will be controlled by a combination of physical and chemical factors including solubility; sorption; radionuclide distribution in the waste and its accessibility; water flow rates and flow paths. As source terms for mathematical modelling, the release data are expressed in the form of a dimensionless release coefficient (Rc) equal to the concentration of the radionuclide in the leachate divided by the initial concentration of the radionuclide in the waste. The Rc values presented in Table II are based on the long term release values after the initial peak has subsided. Release coefficients calculated from leachate samples taken from the disposal trenches at the Drigg site are also quoted for comparison.

For each radionuclide, the results show good agreement between the different lysimeters and the site data despite differences in size, waste composition and mode of operation. These prototype experiments did not yield any measurable quantities of actinides in the leachate due to the very low concentrations of these elements in typical Sellafield LLW. Subsequent lysimeter experiments have been conducted with artificially spiked waste samples and these results will be reported separately.